Standardize pull request guidance

[konojunya]

Background

Recent Stack pull requests consistently explain their context, implementation, validation, and review concerns, but each author currently has to recreate that structure manually. Repository-local guidance should make the expected format available to every contributor and coding agent.

Implementation Approach

• Add .github/pull_request_template.md with the established four-section structure.
• Add or update AGENTS.md to require English pull request titles and bodies.
• Require contributors and agents to retain and complete every template section.

Testing Approach

• git diff --check
• Verified that the template contains all four required headings in the expected order.
• Verified that AGENTS.md references the repository-local template and explicitly requires English pull request titles and bodies.

Review Perspectives / Points of Concern

• This changes contribution guidance only; runtime behavior, packages, releases, and repository settings are unchanged.
• The template uses HTML comments so its prompts do not add noise to completed pull request descriptions.